Gamma-Ray Bursts:

Signals from titanic Stellar Explosions in the deep Universe

What are Gamma-Ray Bursts?

Cosmic Gamma-Ray Bursts (GRBs) were discovered with the Vela satellites in the late 1960s. The bursts are bright, transient events in the gamma-ray sky, lasting in the order of seconds. Because of their short life time the bursts were difficult to localize with the first generation of gamma-ray satellites. Consequently, it took 25 years until astronomers found out that GRBs are signals from the remote universe. In the early 1990s the BATSE experiment on the Compton Gamma-Ray Observatory (CGRO) demonstrated that the burster lie at cosmological distances. During its 10 years life time BATSE detected more than 2700 bursts coming from random locations on the sky. The isotropic distribution of the bursts on the sky together with their observed brightness distribution was a strong indication of the cosmological distance scale of the burster. But the ultimate proof of the distance scale, i.e., the measurement of individual redshifts, remained to be done. This had to wait for another satellite.

The gamma-ray burst sky as seen by BATSE. An animation of a burst is shown at the NASA web page. For the light curves of all BATSE all bursts klick here.

GRB Afterglows

Stimulated by the results of the BATSE experiment, in the 1990s it was suggested by theoreticians that GRBs should be followed by a long-lasting afterglow. Affected by the huge amount of energy released during a GRB, the interstellar matter in the environment of the burster should be ioinized, electrons be accelerated to relativistic speeds and emit synchrotron radiation. Such an afterglow was indeed detected in the X-ray band with the Italian-Dutch BeppoSAX satellite in 1997. This allowed for the first time to localize a GRB on the sky with arcmin accuracy within a day after the event. Soon after astronomers also found the optical counterpart of the afterglow. Thus, with the possibility to observe GRB afterglows in the optical bands, astronomers were able to measure the cosmological distance (redshift) to individual bursters by means of optical spectroscopy.

The optical afterglow of GRB 970228 was discovered after BeppoSAX and other satellites included in the Interplanetary Network (IPN) had roughly localized the burst on the sky. It represented itself as a rapidly fading 'optical transient' (OT). Klick here for more links and details.

GRBs - the biggest explosions in the unverse

By the end of 1998 astronomers had already measured the redshifts of about a handful of bursters and it was clear that GRBs signal titanic explosions in the universe. But early in 1999 a burst occurred, which energy budget broke all previous records. The burst from January 23, GRB 990123, developed a bright optical afterglow, which could be followed over many days. The cosmological redshift of the burster was soon found to be 1.6, corresponding to a luminosity distance of several Gpc. Most spectacular, the burst was accompanied by an optical flash that reached the 9th magnitude for several seconds. At a redshift of 1.6, this flash must have been as luminous as 10**16 suns in order to be so bright on the sky. Basically, this flash was outshining the entire universe for several seconds. If it had been located in the Andromeda galaxy, it would have been as bright as the full moon. The afterglow of this burst was not that spectacular, however. One hour after the burst it was already fainter than the 17th magnitude, i.e., more than 10000 times fainter than the brightest stars visible by the naked eye. One day after the burst it had faded by further three magnitudes.

The light curve of the optical afterglow of GRB 990123. Visit Andy Fruchter's homepage for more details of Hubble Space Telescope observations. Visit also the NASA web page.

GRBs are non-spherical explosions

Six years after the discovery of the first optical afterglow following a GRB, it is now well established that the bursts do in fact not represent isotropic explosions. Instead, these explosions are beamed into a narrow cone with an opening angle typically in the order of some degrees. Theoretically it has been demonstrated that these jets are escaping the GRB progenitor with a velocity close to the velocity of light. This leads to a strong relativistic aberration. As a consequence of this, the observer at Earth must lie within the jet opening angle in order to detect the burst. Even though this relaxes the energy budget of these explosions considerably, the total energy release in gamma-rays is still in the order of 10**51 erg. This is a huge amount of high-energy radiation that is released during a GRB.

A collimated outflow of matter in the form of jets is a common phenomenon among Active Galactic Nuclei. Shown here is an impressive image of our neighbour galaxy M 87 taken with the Hubble Space Telescope.

The nearest burst: GRB 030329

Four years after the famous burst 990123 another spectacular event occurred. GRB 030329 was discovered by the HETE-2 satellite on March 29, 2003. Thanks to its rapid localization in the X-ray band, the optical afterglow was soon found on CCD images taken around 1 hour after the event. (Klick here for a detailed discovery report.) The afterglow turned out to be extraordinary bright: One hour after the burst it was still brighter than the 13th magnitude! This triggered world-wide activities to observe this phenomenon, including several amateur astronomers. High-resolution spectra of the afterglow taken by our group with the Very Large Telescope (VLT) at ESO some hours after the burst showed that this burst was the closest ever seen. Its redshift was only 0.1685, i.e., the burster was 'only' 800 Mpc away. Therefore it was so bright.

The optical afterglow of GRB 030329 observed with the Tautenburg Schmidt telescope (color composit).

The GRB-supernova connection

Since 1997 nearly two dozen GRB afterglows could be observed over many days. Among the astonishing facts astronomers learned about GRBs was increasing evidence that the bursters are physically related to supernova explosions. At first, there was GRB 980425 which was asscociated with SN 1998bw in a galaxy only 38 Mpc away. But this burst had no detectable afterglow, so some doubt remained that it was a standard GRB or rather a positional coincidence. Then astronomers found evidence for extra light in some late-time GRB afterglows which could be accounted for by an underlying supernova component. The final proof of a GRB-SN connection however required a spectroscopic confirmation of supernova light. This was provided by the Gamma-Ray Burst Afterglow Collaboration at ESO (GRACE) and others in the case of GRB 030329. Find a copy of the GRACE' collaboration Nature paper on this subject here (Hjorth et al. 2003), or for example visit the site of SpaceDaily. See Hjorth et al. (2003) for more details. Read also the papers by Stanek et al. (2003) , Matheson et al. (2003), and Kawabata et al. (2003) in order to learn more about this fascinating Supernova.

Supernova light was appearing in the optical afterglow of GRB 030329 some days after the event. Shown here are spectra taken with the observing power of the Very Large Telescope.

The explosion scenario

Thanks to world-wide activities, there is now mounting evidence that GRBs are physically related to stellar explosions (for a review, visit the home page of Peter Mészáros ). Observational data and theoretical models suggest that the bursts are related to the birth of stellar-mass black holes, when the core of a very massive star collapses. This is contrary to most supernova explosions, which either result in the formation of a neutron star or include the desintegration of a white dwarf. Numerical models suggest that the formation and early growth of the black hole in the collapsing stellar core is accompanied by the creation of two highly energetic jets, which finally escape the stellar surface and disrupt the star.

Results of a numerical calculation demonstrating the propagation of a relativistic jet escaping the collapsing core of a massive star. Visit Andrew MacFadyen's home page for more details.

The future of GRB research

GRB research will remain a main stream in observational and theoretical astronomy for the next years. Some of the big open questions for the coming years are: What are the largest redshifts of the bursts? Why do some bursts have only very faint afterglows? Is there a time delay between a GRB and its underlying supernova explosion? Do all GRBs have detectable host galaxies? What is the nature of the short bursts? While currently the dedicated HETE-2 satellite is providing nearly one well-localized GRB per month, the future will bring the dedicated Swift satellite (expected launch in May 2004), and two years later the large gamma-ray observatory GLAST. The number of well-localized GRBs will then increase to more than 100 per year. In order to manage this large data stream robotic telescopes are required. The REM telescope (for a review click here) at La Silla, Chile, Super-Lotis at Kitt Peak, Arizona, ROTSE III, operated at several places in the world, and the Goettingen Monet, are only some of them.

The robotic 0.6-m Super-Lotis telescope at Kitt Peak. A big list of robotic telescopes is maintained by Rick Hessman.

page maintained by S. Klose, Thüringer Landessternwarte Tautenburg.

Last modified: November 7, 2003